The present invention relates to methods for producing 2′,3′-dideoxyinosine useful as an antiviral agent, represented by the following formula (7), (which is called didanosine (DDI) and hereinafter referred to as “DDI”), intermediate compounds that are essential in producing the DDI, and methods for producing the intermediate compounds.

DDI is useful as an antiviral agent and has already been approved as an anti-AIDS drug in many countries including the U.S.A., Japan and European countries.
To obtain a dideoxy (DD) derivative from nucleoside, there is conventionally known, for example, a method where hydroxyl groups at the 2′- and 3′-positions of nucleoside are subjected to thiocarbonylation, followed by radical reduction to form a didehydrodideoxy (D4) derivative, and the D4 derivative is subjected to hydrogenation or the like, thereby obtaining a dideoxy (DD). Some synthesis methods for various antiviral agents based on the above-mentioned technique are reported, which include a method described in: Chu, C. K. et. al. J. Org. Chem. 1989, 54, 2217-2225. However, the method described in the aforementioned literature requires a step of protecting a hydroxyl group at the 5′-position of the nucleoside in advance. For example, when adenosine is used as a raw material for the production of the DD derivative, tert-butyldimethylsilyl group (e.g., refer to Chu, C. K. et. al. J. Org. Chem. 1989. 54, 2217-2225) and trityl group (e.g., refer to Yurkevich, A. M. et al. Tetrahedron, 1969, 25, 477-484) are adopted as the protective groups. However, when DDI is produced using inosine as a raw material, the aforementioned protective groups cause the problems shown below. Namely, as for the tert-butyldimethylsilyl group, it is expensive and a fluorine-based reagent becomes necessary in the process of deprotection. The use of trityl group prevents the progress of the reaction with satisfactory yields (e.g., refer to Japanese Patent Unexamined Publication (JP Kokai) Hei 07-109290). In light of the above, there is an increasing demand for development of methods for producing DDI (7) and 2′,3′-didehydro-2′,3′-dideoxyinosine (4) (which is called D4 inosine and hereinafter referred to as “D4I”) inexpensively so as to obtain satisfactory yields.

There is known a compound where amino group and hydroxyl group respectively at the 1-position and the 5′-position of inosine are protected by benzyl (e.g., Luzzio, F. A. et al. J. Org. Chem., 1994, 59, 7267-7272). However, nothing has been known about a production method for the DDI from the above-mentioned compound as a raw material by subjecting two hydroxyl groups at the 2′- and 3′-positions to deoxylation.